docs: add guide for creating custom nodes and connectors

[Aditya-y9]

📝 Description of the Change

Fixes #272

Summary

This adds two new documentation pages — a guide for creating custom nodes and a guide for
creating custom connectors in Nodify — and links them from the existing docs so users can
easily discover them.

What is this pull request about?

Changes

• Custom-Nodes.md: A beginner-friendly, step-by-step guide that shows how to build a
  fully custom node from scratch using MVVM (ViewModel + DataTemplate). Covers location
  binding, connector anchors, and the fact that any WPF control can be a node.
• Custom-Connectors.md: A guide for customizing connector appearance (ConnectorTemplate),
  header layout (HeaderTemplate), and dynamic behavior with DataTriggers. Includes
  performance and interaction notes.
• Updated _Sidebar.md, Nodes-Overview.md, Connectors-Overview.md, and Getting-Started.md
  to link to the new pages.

🐛 Possible Drawbacks

Are there any possible side-effects or negative impacts with this code change?

No negative impact — this is a docs-only change. The risks are minimal:

• Dead links: If the new .md files are ever renamed, the cross-references in _Sidebar.md, Nodes-Overview.md, Connectors-Overview.md, and Getting-Started.md would need updating. Standard maintenance.
• Outdated patterns: The examples use raw INotifyPropertyChanged rather than a community MVVM toolkit. This is consistent with the existing docs (e.g., Getting-Started.md), so no regression.
• No library code touched, so no behavioral impact on users who don't read the new docs.